Polycarbonate resins are useful materials valued for their physical and optical properties. Methods for the preparation of polycarbonate resins include interfacial processes and melt processes. In interfacial processes, as described, for example, in U.S. Pat. No. 4,360,659 to Sikdar, a bisphenol is reacted with phosgene in the presence of solvents. In melt processes, as described, for example, in U.S. Pat. No. 3,153,008 to Fox, a bisphenol is reacted with a diaryl carbonate. Melt processes are presently preferred because they avoid the use of phosgene and solvents.
Use of a melt process for polycarbonate synthesis requires an industrially efficient process for producing diaryl carbonates. There are several known processes for producing diaryl carbonates. One example of such a process is described by U.S. Pat. No. 4,182,726 to Illuminati et al. In this process, diaryl carbonates are produc by reacting dialkyl carbonates with aryl hydroxides (see Scheme I, below). 
U.S. Pat. No. 4,182,726 also demonstrates that diaryl carbonates can be reacted dihydric phenols to produce polycarbonates (see Scheme II, below). 
A preferred process for making dialkyl carbonates is illustrated in Scheme III, below, and described, for example, in U.S. Pat. No. 5,527,943 to Rivetti et al.; and U.S. Pat. No. 4,218,391 and U.S. Pat. No. 4,318,862 to Romano et al. 
U.S. Pat. No. 5,527,943 (the '943 Patent) also describes a known drawback of the dialkyl carbonate process according to Scheme (III): it produces water as a by-product. Also, hydrochloric acid (HCl) may be continuously added to the reaction mixture to maintain a desired molar ratio of chloride to copper. Therefore, HCl, CuCl catalyst, and water are typically found in the stream exiting the reactor vessel. Hydrochloric acid and copper chlorides are very corrosive in the presence of water, so equipment made from corrosion-resistant materials, such as glass-lined vessels, must be used in the reaction section of a chemical plant making dialkyl carbonates by this process. As corrosion-resistant equipment is expensive, there is a desire to minimize its use in the plant.
A plant for preparing dialkyl carbonates according to Scheme III may contain three sections: a reaction section for converting raw materials to dialkyl carbonate, a separation section for isolating the dialkyl carbonate from unreacted raw materials and by-products, and a purification section for removing water and further isolating the dialkyl carbonate. If the reaction is run with an excess of carbon monoxide, the separation section may contain a subsection, hereinafter referred to as the “cold wash unit”, for removing organic impurities from the mixture of carbon monoxide and carbon dioxide exiting the reactor. The mixture of carbon monoxide and carbon dioxide may then be subjected to further separation and/or purification so that carbon monoxide can be recycled to the reactor.
The '943 Patent teaches that one can minimize the amount of corrosion-resistant equipment required by removing the HCl from the process stream immediately after the reaction section. This eliminates the necessity of using expensive corrosion-resistant materials in the separation and purification sections of the plant. The '943 Patent further suggests that removal of HCl and copper halide salts from the stream immediately after the reaction section can be accomplished by exposing the gas-liquid mixture produced by the reaction to a liquid stream consisting of one of the process fluids.
In view of the above, it was desirable to construct a plant wherein the HCl and any copper halide salts would be removed from the stream after the reaction section to avoid corrosion in downstream sections of the plant. However, a technique similar to that described by the '943 Patent—removing HCl and copper salts by treatment of a vaporized feed in a column using a counterflowing azeotrope fluid from the reaction mixture—failed to prevent corrosion in the cold wash unit.
There is therefore a need for a dialkyl carbonate process that recognizes and eliminates the cause of corrosion in the cold wash unit.